1. Field of the Invention
The present invention relates to an apparatus and a method for preventing data read error for orthogonal frequency division multiplexing (OFDM) symbol demodulation.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating a transmitter of a conventional OFDM communication system.
Referring to FIG. 1, the transmitter of a conventional OFDM communication system includes an audio or video source 110, an audio/video (A/V) encoder 120, a transmission modem 130, a radio frequency (RF) unit 140, and a transmission antenna 150. The audio or video source 110 generates signals. The A/V encoder 120 performs source coding for the generated signals from the audio or video source 110. The transmission modem 130 performs OFDM modulation and channel coding to be tolerant of signal distortion in a wireless space. The RF unit 140 performs frequency conversion and signal amplification to readily transmit signals. The transmission antenna 150 efficiently transmits a signal over a radio space.
FIG. 2 is a block diagram illustrating a receiver of the conventional OFDM communication system.
With reference to FIG. 2, the receiver of the conventional OFDM communication system includes a receiving antenna 210, an RF unit 220, an analog/digital (A/D) converter 230, a receiving modem 240, an A/V decoder 250, and an audio/video player 260. The receiving antenna 210 efficiently receives a transmitted RF signal. The RF unit low-noise amplifies the RF signal and converts the frequency of the signal. The A/D converter 230 converts an analog signal into a digital signal so that a modem processes signals. The receiving modem 240 performs signal processing such as OFDM demodulation and channel decoding. The A/V decoder 250 performs source decoding for an audio or video signal. The audio/video player 260 plays the decoded audio or video signal.
In a fixed communication environment or a mobile communication environment in which a location of a terminal is changed, what greatly influences receiving performance of a receiver using OFDM is a frequency offset and accuracy of time synchronization. Accordingly, there is frequency and time synchronization arrangement for a receiving modem construction for OFDM.
FIG. 3 is a block diagram illustrating a receiving modem shown in FIG. 2.
Referring to FIG. 3, a data buffer 2401 stores an input signal for each frame. A data frame time synchronization unit 2402 acquires approximate time synchronization of the input signal using stored data. An integer frequency synchronization unit 2403 acquires integer frequency synchronization using the time synchronized signal. Next, a symbol time synchronization unit 2404 acquires accurate time synchronization in a ±½ sample. A time synchronization compensating unit 2406 acquires time synchronization of a phase reference signal in the next coming data frame using the acquired time synchronization. There after, an output signal of the time synchronization compensating unit 2406 is input to an I & Q demodulator 2407. An output signal of the integer frequency synchronization unit 2403 is input to a fractional frequency synchronization unit 2405. The fractional frequency synchronization unit 2405 is operated every OFDM symbol in order to compensate input carrier frequency variation due to the Doppler effect occurring in an environment of a mobile receiver and irregular frequency variation occurring due to the short stability of a reference oscillator frequency of a receiver A I&Q demodulator 2407 divides each OFDA symbol into an I signal and a Q signal to compensate a desired frequency offset for incoming data. Signals with compensated offsets are then demodulated by an OFDM demodulator 2409 and decoded by a channel decoder 2410.
It is very important to design a system arrangement to process continuously input RF data in real time in an OFDM communication system. So as to process the continuous input RF data, the processing time for a data frame is required to be shorter than a length of the data frame. Accordingly, a double buffer shown in FIG. 4 and a data buffer having a construction of a ring buffer 420 shown in FIG. 5 may be used for real time processing of input data to the receiver.
With reference to FIG. 4, the double buffer firstly stores input data to a buffer element 1 310, and the modem then processes previously stored data from the buffer element 1 310 while the double buffer stores next data therein to the buffer element 2 320. A usable memory space is limited in the double buffer because data buffering is performed on a fixed-size buffer element basis. Moreover, the double buffer may deteriorate a performance of the receiver when a starting part of a data frame is changed through the influence of time synchronization offset.
Referring to FIG. 5, since a construction of the ring buffer 420 uses one continuous buffer space, there is no restriction of a location to be read and stored in the buffer space. Accordingly, it is easier to compensate time synchronization in the ring buffer 420 structure than in the double buffer. While reading necessary data for a demodulation procedure, there may be an error of storing new data or of reading data that were not completely written. In order to prevent the occurrence of such an error, a data write processor 410 writes data in the ring buffer 420, and increases a write pointer W by the length of the written data. A data read processor 430 uses a ring buffer management method, which reads the data, and increases a read point R by the length of the read data. Further, the data write processor 410 and the data read processor 430 manages to return the increased write point and read point to original positions using a loop counter L. Consequently, when the data write processor 410 writes the data in a memory or the date read processor 430 reads the data from the memory, a writable space, a readable space, and an error state may be checked through the relation between the write point, the read point, and the loop counter. Upon reading of necessary data from the ring buffer 420, an erroneous data read error may be checked. However, time synchronization offset occurring due to the reference clock difference between a transmitter and a receiver may not be compensated. As a result, since a memory space reading data for OFDM symbol demodulation overlaps with a memory space for writing data, the erroneous data read error occurs.